1. Field of the Invention
The present invention relates to a polyester fiber of easy dyeability, and more particularly the invention relates to a polyester fiber containing polypropylene terephthalate/polyethylene terephthalate (PPT/PET) copolyester.
2. Description of the Prior Art
Polyethylene terephthalate (PET) has high strength and good stain resistance, which has been widely and extensively used in the textile field. However, PET has poor dyeability and unmodified PETs can only be dyed with a disperse dye at an elevated temperature of more than 130.degree. C. Such a high temperature requires equipment that can withstand high pressure, which inevitably will cause an increase in the production cost. It is known that the dyeability of PET fibers or fabrics can be improved by using a dye carrier. However, it is also known that the dye carrier will remain in the dyeing waste water, fiber, and fabrics after being processed, which can cause profoundly adverse influence to the environment. Furthermore, the above-mentioned high pressure dyeing process also contributes to the increase of the production cost.
Polypropylene terephthalate (PPT) has the advantages of having the high elastic resilience of nylon and chemical resistance of PET. Also, PPT has better dimensional stability and dyeability than PET and PBT (polybutylene terephthalate) . In the absence of a dye carrier, PPT can still be dyed in rich colors continuously with a disperse dye in boiling water under normal pressure. Having the above properties, PPT has gradually replaced nylon as the material of choice for fabricating such textile materials as the carpet.
In order to improve the dyeability of PET, researchers have tried to blend PPT of good dyeability with PET. For example, a Japanese Patent Publication (Kokai) No. 11-93022 discloses a polyester composite fiber that includes a core made of PET and a sheath made of PPT. The core also contains an antistatic agent. Such a core/sheath composite fiber can be dyed with a disperse dye at a temperature lower than 110.degree. C. while maintaining good heat setting and high antistatic properties. In order to produce such fiber, the sheath portion and the core portion need to be prepared first and followed by a melt spinning process.
In addition, another Japanese Patent Publication (Kokai) No. 59-211620 discloses a process for preparing a polyester flat yarn used for producing high twist textile. In this process, PET, PPT, and/or PBT are blended and melt-spun, which are then subjected to roller extension for preparing the polyester flat yarn.
There have also been many efforts made to produce the PPT/PET copolyester, wherein the main process employed comprises an esterification reaction and a polycondensation reaction. Nevertheless, a polyester fiber containing PPT/PET copolyester has not been produced yet.
Furthermore, dimethyl terephthalate (DMT) is conventionally used as the monomer in the esterification reaction for producing the PPT/PET copolyester. The problem here, however, is that methanol by-product is typically difficult to be recovered and that DMT has always been a very expensive compound. Methods for producing the PPT/PET copolyester can be seen in JP53094393A (1978) of Japan; Ponnusamy and Balakrishnan, J. Macromol. Sci.-Chem., A22(3), pp.373-378 (1985) of India; and U.S. Pat. No. 5,340,909 (1994). Ponnusamy and Balakrishnan synthesize a PPT/PET copolyester by reacting dimethyl terephthalate (DMT), ethylene glycol, and 1,3-propanediol in a melt-polycondensation reaction. However, the obtained PPT/PET copolyester has too small a molecular weight, and the maximum intrinsic viscosity ([.eta.]) is only 0.4 dL/g (solvent: o-chlorophenol, 30.+-.0.1.degree. C.) with no practical value.
DuPont Company in U.S. Pat. No. 5,840,957 (1998) and U.S. Pat. No. 5,849,849 (1998), on the other hand, produces bis(3-hydroxypropyl) terephthalate (BHPT) via a transesterification reaction using lanthanum beta-diketone as the catalyst. Then, BHPT can be polymerized in an inert gas under atmospheric pressure to form PPT, wherein large scale production equipment capable of processing a large amount of flowing nitrogen gas are required thus increasing the equipment cost. Further, the reaction system used in this process is different from the conventional reaction system for producing polyester. Therefore, the conventional system for producing polyester can not be directly adapted for producing PPT by this process.
Chisso Company, disclosed in JP 06002282A (1994), uses a 2,2-alkyl substituted 1,3-PDO, such as 2-butyl-2-ethyl-1,3-PDO, to modify PET. The dyeability can then be improved by using a disperse dye. However, such a 2,2-alkyl substituted 1,3-PDO monomer is not easily accessible in the market; therefore, this technique is difficult to be applied in the conventional polyester field.
Yang Ho Park et al. produce PPT/PET copolyester by reacting PET oligomer and 1,3-propanediol (1,3-PDO, also abbreviated as PG) in a polycondensation reaction (Journal of the Korean Fiber Society, Vol. 36, No. 7, 1999). Theoretically, the alcohol content of the PET oligomer should be reacted with 1,3-propanediol by an interchange reaction and then by a polycondensation reaction to produce a PPT/PET copolyester. The ethylene glycol by-product formed during the interchange reaction must be removed, which adversely increases the cost of production.
From the above conventional techniques, it is known that a fiber containing a mixture of PPT and PET has been developed, however, no dyeable polyester fiber containing a PPT/PET copolyester has been developed yet.